Makefile: support parallel testing
Makefile: support parallel testing

These changes allow "make -j check" to work, which substantially
decreases the testing time here.

Signed-off-by: Rob Browning <1dc5917c2adeeba13d1ef33dbb40691a1b67cc65@defaultvalue.org>

mhoeher/bup,gevaerts/bup,mhoeher/bup,pombredanne/bup,tjanez/bup,jbaber/bup,tjanez/bup,jbaber/bup,pombredanne/bup,tjanez/bup,gevaerts/bup,mhoeher/bup,ToxicFrog/bup,jbaber/bup,gevaerts/bup,pombredanne/bup,mhoeher/bup,ToxicFrog/bup,gevaerts/bup,mhoeher/bup,mhoeher/bup,tjanez/bup,jbaber/bup,gevaerts/bup,pombredanne/bup,jbaber/bup,tjanez/bup,pombredanne/bup,ToxicFrog/bup,ToxicFrog/bup,jbaber/bup,pombredanne/bup,ToxicFrog/bup,tjanez/bup